Gmail - send mail as

I have an ISP smtp server that I'm happy to use and is super fast of course.

I have a separate account who's MX records are gmail, so while it looks like I have external@account.com, I just go to gmail to use the account.

If I add myname@gmail.com as a gmail account, I get all the features of Zimbra's syncing like contacts and calendar which are important - but I'd like to modify the "send mail as" settings, and I figured the only way to do that would be to add Gmail as an IMAP account. But, when I do that, I don't get all the calendar and contact synching.